Whether or not Kizaru gets defeated during this arc or not will have to wait and see(If Oda actually goes through with it which I wouldn't be surprised), I still believe we are going to see Luffy vs Kizaru.
With all these different excuses I'm seeing, it seems like some people are nervous about the possibility of Kizaru being defeated during this arc.
Most people have this headcanon that all the Admirals including Kizaru have to be saved for the final arc or for some other people only Sanji should fight Kizaru and not Luffy, but there is always that slight chance that Oda might not do that and instead he might have Kizaru get taken out a bit earlier.
Also you are right, no arc had a Admiral be the main villain. You could say that KIzaru was kind of the main villain during the Sabaody arc but as you said that arc was more of a transition arc.
With this arc probably lasting a bit more longer and with Kizaru showing up, it's obvious that Kizaru will most likely be the main villain of the Egghead Island arc.